Biological membranes such as cell membranes are intrinsically impermeable to ions and polar molecules. Essential transport of ions through a membrane is achieved by channel or pump proteins. The detailed structure of these proteins and their mechanisms in directing ion traffic through pores in the membranes are not completely known. Two types of channels have been identified, one operated by membrane depolarization (examples: sodium and potassium channels) called voltage-gated, and a second operated by receptor binding (such as acetylcholine), known as ligand-gated. The flow of ions through the pores in the membranes can be assessed by electrical conductance measurements. The study of single-channel ion flow is important to the basic understanding of membrane permeability. It also has practical applications in health, chemical sensing, and agriculture in that molecules that block membrane traffic, and that may be useful as pesticides or as a human deterrent, can be identified. Single-channel ion flow can be studied on cell membranes directly or by the use of artificial lipid bilayers and pore forming proteins such as alpha hemolysin (α-HL) and peptaibol alamethicin.
Currently, conductance measurements across cell membranes are carried out by the conventional patch-clamp amplifier method. In this technique, a cell is drawn against a glass pipette tip (about 1 μm in diameter) so as to form a tight seal. Generally, the seal must be sufficiently tight that the current path around the seal has a very high resistance (e.g., in the giga-ohm range). The electrical current flowing through the membrane that separates solutions in the pipette and outside is then measured by a conventional electrical circuit. This electrical current is influenced by the flow of ions through a single channel and by its states (gate closed or open). While this method allows monitoring of single channels with a time resolution of microseconds, it is labor and skill intensive and is not suitable for high throughput screening. The conventional patch-clamp amplifier operates from DC to a frequency of 10–100 KHz, and is limited by the greater than 1 giga-ohm input impedance required to convert femptoamp channel currents to readable voltages. Because the technique measures very low levels of current flowing across the membrane, the signal being measured is inevitably associated with significant noise. Conductance measurements are also carried out on planar lipid bilayers, also known as black lipid membranes (BLMs). These are artificial membranes suspended in a small (e.g., 0.1 mm) aperture in which proteins are situated. The patch clamp amplifier apparatus is then used to measure current/voltage characteristics of a single protein or several situated in this membrane as a function of time, ligand binding, etc.